The basic principle of such motors is to transfer the inlet fluid at a pressure P1 into a variable-volume intermediate chamber which, after inlet valves have closed and exhaust valves have opened, empties to an outlet as a fluid flowing at the same rate but at a pressure P2 which is less than the pressure P1.
The overall energy used by such hydraulic motors is a function of the fluid flow rate and of the difference between the pressures P1 and P2.
A very important component part in a device of this type is the assembly constituted by its inlet and exhaust valves and by the control system therefor.
In general, present devices use slide valves or mushroom-shaped poppet valves. This gives rise to systems that are complex.